Grain oriented sheet metal having a vanadium nitride dispersion



United States Patent Ofi ice 3,184,346 Patented May 18, 1965 3,184,346GRAIN ORlllENTED SHEET METAL HAVING A VANADIUM NITRIDE DISPERSIQN HowardC. Fiedler, Niskayuna, N.Y., assignor to General Electric Company, acorporation of New York No Drawing. Original application Jan. 4, 1960,Ser. No.

60. Divided and this application Apr. 27, 1961, Ser- No. 105,899

1 Claim. (Cl. 148-3155) This invention relates to magnetizable iron andrelated alloys such as those used in transformers, motors, etc., andmore particularly to grain oriented polycrystalline sheet-like bodiescomposed principally of an alloy of iron and silicon containing somevanadium and nitrogen.

This application is a division of copending application Serial No. 60,filed January 4, 1960, and assigned to the same assignee as the presentinvention.

T he sheet materials to which this invention is directed are usuallyreferred to in the art as electrical silicon steels or, more properly,silicon-irons and are conventionally composed of iron alloyed with about1.5 to 6.0 percent and preferably about 2.5 to 3.5 percent silicon andrelatively minor amounts of various impurities, such as sulfur,manganese, phosphorous, and having low carbon content as finishedmaterial.

Such alloys crystallize in the body-centered cubic crystallographicsystem at room temperature. As is well known, this crystallographicarrangement refers to the symmetrical distribution or arrangement whichthe atoms forming the individual crystals or grains assume in suchmaterials. The body-centered cube is composed of four atoms, eacharranged at the corners of the unit cube with the remaining atomspositioned at the geometric center. Each unit cell in a given grain orcrystal in these materials is substantially identical in shape andorientation with every other unit cell comprising the grain.

The unit cells or body-centered unit cubes comprising these materialseach have a high degree of magnetic ani sotropy with respect to thecrystallographic planes and directions of the unit cube and, therefore,each grain or crystal comprising a plurality of such unit cells exhibitsa similar anisotropy. The silicon-iron alloys to which this invention isdirected are known to have their easiest direction of magnetizationparallel to the unit cube edge, their next easiest directionperpendicular to a plane passed through diagonally-opposite parallelunit cube edges, and

their least easiest direction of magnetization perpendicularv to a planepassed through a pair of diagonally-opposite atoms in a first unit cubeface, the central atom and a single atom in the unit cube face which isparallel to the first face.

It has been found that these silicon-iron alloys may be fabricated byunidirectional rolling and heat treatment to form sheet or stripmaterial composed of a pluarity of crystals or grains, a majority ofwhich have their atoms arranged so that their crystallographic planeshave a similar or substantially identical orientation to the plane ofthe sheet or strip and to a single direction in said plane. Thismaterial is usually referred to as oriented or grain orientedsilicon-iron sheet or strip and is characterized byhaving 50 percent ormore of its constituent grains oriented so that four of the cube edgesor unit cells of the grains are substantially parallel to the plane ofthe sheet or strip and to the direction in which it was rolled and a(110) crystallographic plane substantially parallel to the plane of thesheet.

It will thus be seen that these so-oriented grain have a direction ofeasiest magnetization in the plane of the sheet in the rolling directionand the next easiest direction of magnetization in the plane of thesheet in the transverseto-rolling direction. This is conventionallyreferred to as cube-on-edge orientation or the (110) [001] texture. Inthese polycrystalline sheet and strip materials, it is desirable to haveas high a degree of grain orientation as is attainable in order that themagnetic properties in the plane of the sheet and in the rollingdirection may approach the maximum attained in single crystals in thedirection. Strip and sheet grain oriented siliconiron alloys have beenpreviously used as transformer core materials, electric motor andgenerator laminations and in other electrical and electronicapplications where the high degree of electromagnetic properties in therolling direction of the sheet or strip may be employed advantageously.For most applications, the highest degree of grain orientation ortexture obtainable is desirable. Usually, materials having more thanabout 70 percent of their crystal structures oriented in the [001]texture are considered to have a strong texture.

Heretofore, the cube-on-edge texture has been produced in silicon-ironalloys by adding controlled amounts of manganese and sulfur, introducedinto the material as impurities. The manganese and sulfur are probablypresent as a dispersion of manganese sulfide. The significance of themanganese sulfide is demonstrated by the fact that strong texturescannot be developed in high purity vacuum melted silicon-iron alloysprepared from substantia ly pure iron and silicon and processed in thesame manner as alloys containing manganese sulfide.

Before these silicon-irons may be used in certain applications, such astransformers, etc. mentioned above, it is necessary to removesubstantially all of the sulfur in order to obtain optimum magneticproperties, since the presence of sulfur exerts an adverse effect uponthese magnetic properties.

In actual steel mill practice, cube-onedge materials are prepared bycasting ingots from alloys containing from about 2.5 to 4.0 percent, andpreferably from 2.5 to 3.5 percent by weight silicon, less than 0.035Weight percent carbon, about 0.02 to 0.03 weight percent sulfur, andless than 0.1 weight percent manganese. These ingots are conventionallyhot worked into a strip or sheet-like configuration, usually less than0.150 inch in thickness, referred to as hot rolled band.

The hot rolled band is then cold rolled with appropriate annealingtreatments to the finished sheet or strip thickness, usually involvingat least a 50 percent reduction in the thickness, and given a final ortextureproducing annealing treatment. As presently practiced, this finalanneal is accomplished in two steps. First, a short normalizing annealis carried out at about 800 C. for about five minutes in a wet hydrogenor wet cracked gas atmosphere. This anneal serves at least twopurposesit decarburizes the material or, stated otherwise, reduces thecarbon content of the material to a value of less than 0.010 percent byweight, and additionally causes the worked metal structure torecrystallize into a fine grain microstructure. This is usually referredto as a primary recrystallization. Because of the relatively lowtemperature and short time involved in this anneal, it is possible toemploy a continuous annealing technique wherein the sheet or strip ofmetal is fed through a controlled atmosphere furnace at a rate such thatany given portion of the strip is raised to the required temperature forthe necessary period of time. Such continuous annealing techniques arewidely employed in the metallurgical arts and are usually moreeconomical than batch anneals.

The decarburized strips or sheets are then cooled and coated with arefractory material and, depending upon their size and configuration,either coiled or stacked and placed in an enclosed box which is providedwith an atmosphere of dry hydrogen or. dry cracked gas or'in acontrolled atmosphere furnace and annealed therein.

During this anneal, two actions occur. First, a secondaryrecrystallization takes place wherein the small grains havingthe'desir'ed (100) [001] orientation grow at the expense of grainshaving other orientationsand, secondly, the sulfur content is loweredand preferably substantially removed. As commercially practiced, it hasbeen found necessary to anneal such material over a considerable periodof time in order to accomplish the .two actions the ingot initially,then thenal annealing atmosphere must be controlled to prevent prematureremoval of the nitrogen, particularly if only'a small amount of vanadiumnitride was originally present.

The procedure followed in the present invention to produce asilicon-iron body having the desired orientation is to cast the moltenmetal containing as little sulfur and manganese as possible, into ingotor slab form. It

' will be appreciated that traces of sulfur and manganese previouslystated and to produce acceptably strong textures. This has required theemployment of a batchtype anneal, the total time required for suchannealing usually extending from one to two days, since in order alloymust be heat treated, at moderately high temperatures, e.g., 1175 C. forabout one hour. Obviously, the extreme length of the heating periodmaterially increases the cost of producing the final sheet, both'throughthe cost of the operation itself and by resulting in a batch process.

It is a principal object of this invention to provideironbase siliconalloy strip which can be processed in shorter periods of time and atlower temperatures than can existing iron-silicon alloys to producesheet or strip material having a preferred cube-on-edge grainorientation.

, Another object of this invention is to provide a process for treatingan iron-base siliconalloy containing vanadium to precipitate a vanadiumnitride phase promoting development of the preferred cube-on-edge grainorientation.

Other objects and advantages. of the present invention will be in partobvious and in part explained by reference to the accompanyingspecification.

Briefly stated, the present. invention utilizes relatively smalladditions of vanadium in silicon-iron electrical. steels forprecipitation as vanadium nitride both in the ingot and when the metalis subjected to an annealing treatment within a nitriding atmosphere.The vanadium nitrides are effective in controlling the secondaryrecrystallization to develop a preferred grain orientation and are alsoreadily removed from the finished material after the preferredorientation has been developed.

' More specifically, it has been found that small additions of vanadiumup to 0.15 weight percent, andas little as 0.05 weight percent, to themelt will provide a suflicient amount of vanadium nitride to enablesubsequent development of a strong preferred orientation when thesolidified melt, after processing to strip form, is given a finaltexture-developing anneal. A greater amount of vanadium can be present.in the alloy but it is not necessary to the process. As little as 0.002weight percent nitrogen will generally form enough nitride to beeffective. The nitrogen present in the. ingot is believed to be largelypresent as vanadium nitride, the' uncombined vanadium remaining in solidsolution. Generally, the .vanadiumnitride dispersion may be formed inthe metal either by placing sufiicient quantities of vanadium andnitrogen in the original melt or by subjecting a will probably be foundin the alloy due to the impurities in the raw materials or from therefractory furnace crucible V 7 Upon solidification of the metal, it ishot rolled to about 100 mil thickness, this particular thickness usuallybeing referred to as the hot rolled band. The hot rolled "bandisfpermitt'ed towel and then cold rolled-to within "the range ofthicknesses of from 0.029 inch to about 0.025

inch and then given an intermediate normalizing heat treatment. ,Themetal is then cold rolled to 12 to 14 mil thickness, and'fin'alannealing done to eifect'secondary recrystallization, that is, to bringon the (110)[00 l] orientation. The final anneal'to develop the textureis advantageously carried out by heating the metal as rapidly aspossible toa temperature between. 950 C. and .1050" and then holding attemperature for 10 to 20 minutes to develop the texture or by heatingitto'a temperature between 1050 C. and 1150" C. at a moderate rate, suchas about 7 C. per minute. 7

According to the present invention, the atmosphere or,

environment in which the metal is given its final texturedevelopinganneal is important since, in some instances, the nitrogen already inthe strip should be retained, while in other instances,-sorne additionalnitrogen should be added to form more. particles of vanadium nitride.The particles formed by combination of the vanadium remainingin solutionand the nitrogen from the atmosphere are particularly effective inpreventing normal grain growth. Lack of control 'of the annealingenvironment could resu lt in absence of the second phase prior to developrnent 0f the desired texture, thereby permitting normal glfllll;growth and lowering of the amount of texture 7 obtained;

- To; more clearly illustratethe present invention, several alloys werecast and subjectedto varying conditions to vanadium-containing alloybody to a nitrogen-containing atmosphere during the. final heattreatment. This nitrogen combines with the solid solution vanadium andforms additional vanadiumnitride, which assists development of thetexture. The'function of the vanadium nitride particles is to preventnormal grain growth. Those grains with [001] orientation readily grow inthe fine grain matrix, and result in the development of a strongtexture. Of course, if the nitride phase is present in develop thecube-on-edge orientation.

Table I, following, list's in weight percentages, compositionsof severalalloys containing varying amounts of vanadium and nitrogen:

Table I v In 01: 1 s Heat No. g ana y es s1 0 v s N o 3. 2 0.002 0. l20. 003 0. 002 0. 007 3. 2 003 11 O04 002 009 3. 2 003 12 004 003 006 3.25 g. 005 12 004 003 006 3. 25 g. 005 09 006 003 007 3. 25 g. 005 10.004 004 005 3.25 2.005 .10 .003 .003 .005 3. 25 005 11 003 001 .005 3.25 g. 005 10 006. 004 004 3. 25 g. 005 11 003 006 3. 25 g. 005 07 003003 003 3. 25 g. 005 08 004 002 .004 3. 25 g. 005 l0 001 006 003 3. 252. 005 07 002 004 004 llllonliinial cgifile'nt tbased (anugeightiladdid.

naysso esripin care 1; att enitro en cont 1: rather than 0.004 percent.g en was 0 009 All of the alloysof Table I were made from a basematerial of'electrolytic iron to which was added pure vanadium and '98percent ferrosilicon. The amount of nitrogen the ingot was controlledby' the ratio of nitrogen, to argon in the gas which was blown over thesurface during melting. The ingots were made by pouring into eithergraphite or cast iron molds.

After casing, about one-sixteenth inch was ground off the surfaces ofthe ingots to remove irregularities and they were then heated to between1000 C. and 1100 C. for rolling to 100 mil hand without reheating. Thehot rolled bands were pickled in a dilute hydrofluoric-hydrochloric acidsolution. After a five-minute heat treatment at 900 C. in a hydrogenfurnace, the bands were rolled to an intermediate gauge of 28 mils ifthe final gauge was to be 14 mils, and .25 mils if the final gauge wasto he 12 mils. The intermediate heat treatment was also five minutes at900 C., after which the material was rolled to final gauges.

As previously mentioned, the atmosphere or environment used during thefinal annealing operation is important in its etfect upon the nitrogencontent, and therefore the vanadium nitride content, of the body.Samples taken from Heat 9 were heated for one-half hour at 1020 C. in adry hydrogen (60 F. dew point or less) atmosphere. There was apronounced tendency for nitrogen to be withdrawn from the body and thisis evidenced by the fact that after the heat treating, the nitrogencontent was only 0.0005 weight percent and that during the heattreatment the sample mainly underwent normal grain growth. On the otherhand, samples from the same heat had complete secondary growth whenannealed in a onequarter nitrogen-tluce-quarters hydrogen atmosphere.After one-half hour, samples annealed in this manner had nitrogencontents of 0.008 weight percent, indicating that the atmosphere hadbecome essentially a neutral one so that the nitrogen content of themetal remained substantially unchanged. By increasing the nitrogencontent of the atmosphere to two-thirds nitrogenone-third hydrogen, itbecomes slightly nitriding, and after one-half hour at 1020" C., thenitrogen content of the sample strip was increased about 0.002 percent.The resulting greater number of vanadium nitride inclusions results inthere being comparatively few large secondary grains growing in anotherwise fine grain matrix.

Although the nitrogen is lost within one-half hour at 1020 C. in a dryhydrogen (60 F. dew point or less) atmosphere, a similar heat treatmentin hydrogen with a dew point of 30 F. results in the retention of themajor portion of the nitrogen (0.006 percent, starting with an initial0.009 percent) and complete secondary recrystallization occurs. It isbelieved that the nitrogen is largely retained during heat treating in ahydrogen atmosphere if the dew point is such that the atmosphere isoxidizing to the silicon at the temperature of the heat treatment. Theformation of an oxide film is believed responsible for this occurrence.

Magnetic measurements were made on samples Whose texture was developedeither by heating at a controlled rate through the secondary graingrowth temperature range or by holding for 15 minutes at a temperaturewithin that range. In the first group, designated A in Table II, thesamples, after coming up to 1000 C., were heated at a uniform rate,about 7 C. per minute, in a two-thirds nitrogenone-third hydrogenmixture. The nitrogen content of the samples was then reduced to 0.0005percent or less by holding 10 minutes with hydrogen flowing through theheating furnace. Ten minutes were used to allow time for the retort tobe purged of nitrogen.

The second group of strips, designated B in Table II, were heatedrapidly to 1040 C., held minutes in a two-thirds nitrogenone-thirdhydrogen atmosphere and then for an additional five minutes with onlyhydrogen atmosphere and then for an additional five minutes with onlyhydrogen flowing to allow the nitrogen to be purged. The temperature wasthen raised to 1100 C. and the samples held an additional five minutes.

The following Table 11 sets forth the properties obtained in the A and Bgroup strips processed as ex- K!) plained above. The heat numberscorrespond to the heat numbers set forth in Table I.

The texture percentages listed were obtained by dividing the maximumtorques obtained with a torque magnetometer in a field of 1000 oersteds,by the maximum torque of a single crystal.

Since it is not necessary to use a nitrogen-containing atmosphere orenvironment where the required amount of nitrogen is already present inthe initial alloy, strips from Heats 9, 11, 12 and 13 were processed andannealed in hydrogen atmospheres of varying dew points. Other samplesfrom the same heats were coated with magnesia and annealed in a hydrogenatmosphere having a dew point of 60 F. The strips were all heated from900 C. to 1200 C. at 100 C. per hour in dry hydrogen and in hydrogenwith a dew point of 26 F. The average percent textures obtained on thesestrips are shown in Table III:

The macrostructure of the samples heat treated in the dry hydrogenatmosphere (60 F. dew point) consisted of small secondary grains, plussmaller grains which grew by normal grain growth. The strips heated inthe 26 F. dew point hydr gen consisted entirely of large secondarygrains.

As the results in Table HI indicate, the magnesia coating providesanother method of keeping nitrogen within the body to perform itsfunction. It is therefore possible, with the use of a magnesia coating,to retard the loss of nitrogen and to do conventional batch annealing,either as coils or laminations.

Strip material was also prepared from ingots of Heats l, 2 and 3 (TableI) by heating the ingots to about 1000 C. and rolling, withoutreheating, mil thick hot rolled band. This rolled material was thenannealed at 900 C., for from 3 to 30 minutes in dry (dew point about 60F.) hydrogen to effect complete recrystallization. This anneal may beomitted if desired and an atmosphere other than hydrogen may be used.The bands were then cold rolled to 25 mil thickness and annealed at 860C. for two minutes in dry hydrogen, then cold rolled to 13 milthickness. It should be noted that this intermediate annealingtemperature should be maintained between about 850 C. and 950 C. foroptimum results.

Strips of this material were then heated rapidly to 1000 C. in anatmosphere consisting of two-thirds nitrogen and onethird hydrogen andsubsequently heated to 1100 C. at a rate of 7 C. per minute in the sameatmosphere. The secondary recrystallization was complete by the time thestrip reached the 1100 C. temperature and the degree of orientation forall heats was between 75 and percent.

C. to avoid cracking and. related rolling difficulties.

Following this treatment, the strips were heated in dry hydrogento atemperature of 1100 C. for about five minutes in order to remove thenitrogen. Use of slightly lower temperatures, for example, 1050 62., re-

quires somewhat longer purification periods, e.g.', up to 15 minutes.

The use of vanadium nitride inclusions hasbeen found effective todevelop cube-on-edge texture in alloys hav- 7 ing a wide. range ofsilicon contents. In Table IV are magnetic properties measured on28centirneter lap-joint Epstein packs of alloys containing 2.0, 3.2 and5.1 percent silicon.

I The final anneal consisted in heating from 900 C. to 1100 C; at25.C.'per hour or 100 C. per

hour, in an atmosphere containing 50' to 67 percent nitrogen, remainderhydrogen. The purification treatment consisted inheating the strip at1100 C. in hydro.- gen for vone hour or less.fIt is necessary to coldroll the higher silicon content'alloys, for example, 4.5 to6.0 percentsilicon, at temperatures of from 200 C. to 350 The advantage of thepresent process is that the nitride inclusions that are used todevelopthe texture are readily removed within five minutes at temperatures'inthe region of 1100 C. in dry hydrogen. The nitrogen content is decreasedfrom about 0.005 percent to 0 0 03 percent by such a heat treatment.Silicon-iron utilizing sulfide inclusions to develop the texturerequires higher purifying tem nitride, on the otherhand, can be removedfrom the final product by subjecting the strip to arhydrogen atmospherefor a short period of time at a temperature as low as 1050 C.

While specific examples of the invention havebeen recited in theforegoing specification, it will be obvious to those. skilled in the artthat various changes and modifications may be made without departingfrom the invention, and it is intended to cover in the appended claimall such changes and modifications that come within the true spirit andscope of the invention.

What Iclaim'as new and desire vto secure by Letters Patenteof the UnitedStatesis:

A polycrystalline sheet-like body having a majority of the constituentgrains oriented in the (110) [001] direction consisting essentially offrom 2 to 6 Weight percent silicon, from about 0.05 to 0.15 weightpercent vanadium, at least 0.00'2weight percent nitrogen combined withsaid vanadium to form a vanadium nitride dispersion promoting said(1l0.) [001] grain orientation direction .within said sheet-likebody,and the remainder substantially alliron.

References Cited by the Examiner.

The Iron Age, vol. 171, No. 6,.February' 5, 1953, pages 7 147-152 and186 (pages 148-149 relied on').

Metals and Alloys, vol. 10, 2631 relied upon.

DAVID L. RECK, Primary Examiner; MARCUS U. LYONS, Examiner.

January-June 1939, pages

